The present invention relates to systems for applying coatings under pressure to a web of material. A variety of coatings may be used, such as, but not restricted to, solvent- or water-based coatings, and the web may be made of a variety of materials, such as, but not restricted to, steel, aluminum, textiles, paper or film. U.S. Pat. No. 5,743,964 “Pankake” is an example of prior art roll coating.
The primary known technology for application of film in the range of 1 milligram per square inch to 30+milligrams per square inch of fluid on a substrate at speeds greater than 250 feet per minute involves a process known as roll coating. This involves picking up a fluid out of an open pan with a pick-up roll or feeding the fluid by gravity into a top nip. (A nip is the pinch point between rollers.) The fluid is then transferred from that roll to the next or is transmitted through a nip to the next roll. Eventually, the fluid is transferred from a roll to the web.
Another approach commonly used for applying fluid to a substrate involves the use of a die or slot. This process is normally limited to speeds up to approximately 200 feet per minute. The fluid may be deposited onto a roll for transfer to the substrate or directly onto the substrate with this method.
Coating being picked up out of a pan, sprayed, or nip fed is exposed to ambient conditions and the atmosphere. This permits dry out or skinning-over and evaporation of volatiles that contribute to product variability and environmental pollution, foaming, and splashing. Numerous other defects are also associated with unstable or uncontrolled fluid dynamics that occur at the entry point of the roll into the fluid contained in the pan, the exit point of the roll out of the fluid in the pan, or at the nip point. Some of these defects are often labeled as skips, seashore, ribbing, blisters, voids, shinnies, or splotching. The fluid picked up out of a pan is susceptible to being slung from the roll ends, creating a safety hazard, product defects, and a mess.
The appearance and thickness of the applied fluid is governed by a very complex relationship between the equipment configuration, equipment settings, and the fluid characteristics. Some of these variables include the number of rolls, direction of rotation of the rolls, roll material, roll finish, roll diameter, roll hardness, roll geometry, nip pressures, fluid viscosity, and fluid rheology. The relationships of all of these variables in the roll coatings process today provide a relatively small window for successful application of a specific fluid at a specific thickness. Fluids are very often applied at viscosities of 10 to 500 centistokes, depending on the desired applied film thickness. This requires the addition of large volumes of solvents or carrier fluid in many cases. The evaporation of these large volumes of solvents into the atmosphere is very undesirable from an environmental standpoint. Also, since the solvents evaporate from the open pan during the process, the characteristics of the coating are constantly changing during the process, making it very difficult to control the process.
The set-up of the above process must also be done in a way to achieve the desired film thickness while minimizing an appearance defect known as ribbing in the roll coating process. Typically, fluids are reduced in viscosity, and long flow-out zones are provided. These flow-out zones permit the ribs to be leveled out.
The use of open pans also creates major limitations to rapid, repeatable product changes. Typically, a product change for a pan feed system requires to between 10 minutes and several hours. To achieve product changes in less than 30 minutes usually requires additional investments of millions of dollars in capital equipment and labor intensive activities on major web processing lines.
As will be seen from the subsequent description of the preferred embodiments of the present invention, these and other limitations and shortcomings of the prior art are overcome by the present invention.